The new tube stock is designed for it to be retrofitted, but replacing all of the signalling equipment is expensive (and likely to require some closures, both o

The new tube stock is designed for it to be retrofitted, but replacing all of the signalling equipment is expensive (and likely to require some closures, both of segments of the line being upgraded and of the whole line). The subsurface lines have interworking with normal mainline services, in some places sharing platforms with them, which makes fitting platform-edge doors very difficult if not impossible.